1. Field of the Invention
The present invention relates to vibrators, and in particular to vibrators used as acceleration sensors and so forth for measuring acceleration in car navigation systems.
2. Description of the Related Art
FIG. 4 is a perspective view illustrating one example of a conventional vibrator. FIG. 5 is a side view illustrating a side of the vibrator. The vibrator 1 includes, for example, a rectangular, plate-shaped vibrating member 2. A supporting member 3 is formed at one end of the vibrating member 2 along the longitudinal direction. The supporting member 3 is connected to the end of the vibrating member 2 and a node portion along the longitudinal direction by thin connecting portions 3a. The supporting member 3 is mounted on a base or the like by using a technique such as soldering. In this arrangement the base and the vibrating member 2 need to be spaced from one another so that the base cannot hinder vibrations from the vibrating member 2. Accordingly, the supporting member 3 is formed to be bent.
Also, a weight 4 is mounted on the other end of the vibrating member 3. The weight 4 is connected to the other end of the vibrating member 2 and the node portion along the longitudinal direction by the other connecting portions 5. The vibrating member 2, the supporting member 3, the connecting portions 3a, the other connecting portions 5 and so forth are formed by punching a permanent elastic metallic material, such as elinver, in a predetermined shape and by bending the punched material.
Piezoelectric devices 6a, 6b and piezoelectric devices 6c, 6d are formed on both surfaces of the vibrating member 2 along the longitudinal direction. The piezoelectric devices 6a and 6b are oppositely formed on the vibrating member 2 in the central longitudinal direction to the end, while the piezoelectric devices 6c and 6d are oppositely formed on the vibrating member 2 along the central longitudinal direction with respect to the other end. The piezoelectric devices 6a, 6b and the piezoelectric devices 6c, 6d include piezoelectric layers, respectively, and electrodes are formed on both surfaces of each piezoelectric layer. One of the electrodes is bonded to the vibrating member 2. The piezoelectric devices 6a, 6b and the piezoelectric devices 6c, 6d are oppositely polarized. For example, when the piezoelectric layers of the piezoelectric devices 6a and 6b are polarized from the exterior to the vibrating member 2, the piezoelectric devices 6c and 6d are polarized from the vibrating member 2 to the exterior.
The vibrator 1 is used as, for example, an acceleration sensor. In this case the piezoelectric devices 6a, 6b and the piezoelectric devices 6c, 6d are supplied with a driving signal in phase and at the same level. Since the piezoelectric devices 6a, 6b and the piezoelectric devices 6c, 6d are oppositely polarized, both pairs are oppositely vibrated by being supplied with the same driving signal. Consequently, the vibrating member 2 has longitudinal vibrations in mutually opposite directions, putting a boundary at its center. In other words, as shown by solid line arrows in FIG. 5, when a portion of the vibrating member 2 on which the piezoelectric devices 6a and 6b are formed lengthens, a portion of the vibrating member 2 on which the piezoelectric devices 6c and 6d are formed shortens. In reverse, as shown by dashed line arrows, when the portion of the vibrating member 2 on which the piezoelectric devices 6a and 6b are formed shortens, the portion of the vibrating member 2 on which the piezoelectric devices 6c and 6d are formed lengthens. Accordingly, when observed from both ends of the vibrating member 2, a portion of the vibrating member 2, having approximately 1/4 of the overall length, becomes a node. In addition, the vibrating member 2 has longitudinal vibrations in mutually opposite directions on both sides of the central portion, thus, also when the vibrating member 2 longitudinally vibrates, its overall length does not change.
When acceleration is perpendicularly applied onto the vibrating member 2 in the longitudinal vibration mode, the vibrating member 2 warps. Then, the weight 4 increases warping of the vibrating member 2. The warp of the vibrating member 2 causes the piezoelectric devices 6a, 6b, 6c and 6d to output signals in accordance with the warp. Consequently, by measuring the output signals from the piezoelectric devices 6a, 6b, 6c and 6d, the acceleration can be detected.
The above-described vibrator has the connection portions formed at one end of the vibrating member and in the vicinity of the node portion, which decreases vibrations leaking from the vibrating member. However, in order to increase warping caused by acceleration, the vibrating member is preferably thin, but a process such as bending causes deformation of the vibrating member. In addition, when the supporting member is soldered to the base, the heat affects the piezoelectric devices and their bonded portions, so that an ideal condition cannot be maintained. As a result, vibrations from the vibrating member leak to the base. When leaking vibrations are conducted back to the vibrating member through the supporting member, vibrations from the vibrating member are affected to cause an increase in temperature drift.